NEAR BAND GAP OPTICAL ABSORPTION IN SEMICONDUCTING VO2

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NEAR BAND GAP OPTICAL ABSORPTION IN

SEMICONDUCTING VO2

P. Merenda, D. Kaplan, C. Sommers

To cite this version:

P. Merenda, D. Kaplan, C. Sommers. NEAR BAND GAP OPTICAL ABSORPTION IN

SEMICONDUCTING VO2. Journal de Physique Colloques, 1976, 37 (C4), pp.C4-59-C4-62.

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JOURNAL DE PHYSIQUE Colloque C4, suppliment au no 10, Tome 37, Octobre 1976, page C4-59

NEAR

BAND

GAP OPTICAL ABSORPTION IN SEMICONDUCTING VO,

P. MERENDA and D. KAPLAN

Laboratoire Central de Recherches, THOMSON-C. S . F.

91401 Orsay, France and

C. SOMMERS

Laboratoire de Physique des Solides, UniversitC Paris XI 91405 Orsay, France

Rbsumb. - On a etudie l'absorption prh du bord de bande dans V 0 2 pur et dans les alliages V I - ~ C ~ ~ O ~ . L'interpretation des resultats indique un lien entre le gap optique et I'dnergie d'activation de la conductivite.

Abstract. - The near band gap absorption has been studied in pure V02 and V I - ~ C ~ ~ O ~ alloys. Interpretation of the results indicates a close connection between the optical band gap and the intrinsic conductivity activation energy.

1. Introduction.

-

Vanadium dioxide (VO,) under- goes a metal-insulator transition at 67 O C . The nature of the electronic states in the insulating phase (M,) has long been a matter of debate : the rutile structure of the metallic phase becomes distorted below the transition temperature and it was not clear whether the metal insulator transition was due to a band gap associated with the distortion [l] or to localization by electron-electron correlations as in a Mott-Hubbard insulator [l41

121.

Recently the situation has been clarified by studies of VO, related alloys [l51 [3, 41 (V1-,M,O,, where M = Cr, Fe, Al) and VO, under applied uniaxial pressure [5]. In these systems one observes phases (M, and T) with different distortion characteristics. A representation of Vanadium displacements, relative to their rutile position, in the M, phase is shown in figure 1. One distinguishes two types of Vanadium chains along the rutile c, axis. On one half of the chains Vanadiums undergo a

pairing displacement, while on the other half they undergo a zig-zag displacement. In the M, phase of pure VO,, all Vanadiums undergo both pairing and

zig-zag displacements (the T phase is intermediate between M, and M,). The crucial fact is that the

zig-zag chains of the M, phase are magnetic which clearly establishes the localized nature of the electrons. In the M, phase, the pairipg on all chains leads to a non magnetic ground state. The picture is there that of electrons strongly localized within the pair, although some delocalization may exist in between the two atoms.

It is now interesting to investigate the optical pro- perties of such a system. In a Mott-Hubbard insulator,

VANADIUM IONS POSITIONS I N A (llO)R PLANE (110) PLANE OXYGEN ION CHAIN 1 OXYGEN OCTAHEDRON 0 0 0

CHAIN 2 CHAlN l CHAIN 2

M2 PHASE

FIG. l. -Vanadium displacements, relative to their rutile positions, in the M2 phase. Diagram in a ( 1 1 0 ) ~ plane. one distinguishes two possible types of optical transitions :

a ) lntrasite transitions, for which the electron is excited on a higher lying orbital on the same atomic site.

b) Intersite transitions for which the electron is transferred on other sites ; the energy of this second type of transitions contains a contribution from the electron-electron correlation energy U.

The present system is slightly more complicated, since one should also envisage intrapair transitions, for which the electron is transferred on the neighbouring site in the same pair.

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C4-60 P. MERENDA, D. KAPZ ,AN AND C. SOMMERS

We have attempted to determine which kind of transitions are involved in the absorption edge of VO,. For this purpose we have studied optical absorption in pure VO, and in V,-,Cr,02 alloys, since the behaviour at transitions between insulating phases with different distortions is expected to be an important clue for the nature of the transitions. For the interpretation of our results, we rely partly on molecular cluster calculations of Vanadium energy levels made by the self consistent Xa scattered wave method [6,7].

In most of the previous optical studies 18, 91 the absorption edge region has not been investigated in detail due to the lack of thin samples of sufficient crystalline quality. Borisov et al. [l01 have studied epitaxial layers but were limited to phonon energies larger than 1 eV. The only detailed investigation was performed by Ladd and Paul [l11 using bulk crystals thinned by polishing. They were able to observe the temperature variation of the transmission edge, but could not analyse its shape, because of the small range in optical absorption coefficient they were able to obtain. The present study is more complete because :

i) reflection was measured together with transmission allowing determination of the absorption coefficient over a larger range in a given sample, ii) samples of different origin and thicknesses were studied, including bulk samples and epitaxial layers, iii) V1-xCrx02 was investigated for the first time.

2. Experimental details. - Absolute reflection and transmission measurements were made, in the 0.6 to 4 p range using a locally designed Strong type spectro- meter [12]. The optical absorption coefficient a was extracted, taking into account multiple reflections in the intensities (interference effects are not important due to small inhomogeneities in the sample thickness). Measurements were performed from liquid nitrogen temperature up to the metal-insulator transition.

For small absorption coefficients, we used bulk crystals prepared either by slow cooling solution growth (VzO, flux) or by vapor phase transport by TeCl,. Thin wafers (- 25 p) were obtained by mecha- nical polishing followed by a chemical etch. For high absorption coefficients, we grew epitaxial layers (1-20 p

thick) on (001) oriented Ti02 substrates by chemical vapor deposition from the chemical system VOC13/H20/H2.

Solution grown bulk crystals and epitaxial layers have a large conductivity jump ( m 105) at the metal

insulator transition. The activation energy in the insulating phase near the transition is 0.42 eV. Some bulk crystals grown by vapor phase transport show a smaller jump and a smaller activation energy (-- 0.1 eV) indicating the presence of donor stoechiometry defects [ 2 ] .

V1 -,Cr,O, alloys were grown in the form of epitaxial layers 1161 using Cr02C12 as a chromium source. Bulk crystals were also grown but were too brittle to be used for optical studies.

3. Optical absorption in pure VOz.

-

Figure 2 shows optical absorption data taken at liquid nitrogen temperature. At low photon energies results obtained from bulk samples are presented. At higher photon ener-

E P I T A X I A L LAYER d = l p P O L A R I Z A T I O N L C R

/'

/

BULK C R V S T I L U N P D L A R I Z E D E X P O N E N T I A L TAIL

/'

E,=88 meV PHOTON ENERGY (.V)

FIG. 2.

-

Absorption coefficient of pure V 0 2 samples versus photon energy at liquid nitrogen temperature.

gies the data are taken from a 1 p thick epitaxial film. There is a slight mismatch between the two curves which may come partly from uncertainties in the film thickness and partly from difference in the polarization of the light reIative to the crystal orientation : At 0.6 eV the absorption coefficient a was found to be 20

%

higher for light polarized perpendicular to the rutile

c, axis than for light polarized parallel to this axis. These results were generally reproducible for different samples, with the exception of the more conductive crystals grown by vapor phase transport which showed a greatly enhanced absorption in the tail region. The general energy dependence of a is relatively smooth,

with a maximum around 1.45 eV. There is no well defined threshold that one can associate with an energy gap. But at low photon energies (h,

<

0.6 eV) one finds a region of exponential behaviour of the form a = A exp(E/E,,) where the energy E. has a value 88 X 10-3 eV independent of temperature. One can identify this region as a tail of extrinsic absorption and tentatively associate an energy gap E, with the photon energy where deviates from exponential behaviour. With this definition a value E, = 0.60 f 0.05 eV is obtained at 77K. Figure 3 shows the absorption coefficient a in the gap region at different temperatures. These data confirm the report by Ladd and Paul [l11 that the shape of the absorption edge does not vary with temperature, i. e. the temperature dependence has the form :

a(E7 T) = f(E - E,(T))

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NEAR BAND GAP OPTICAL ABSORPTION IN SEMICONDUCTING V02

I

3 4 5 6 .7

P H O T O N ENERGY ( s V )

FIG. 3.

-

Absorption coefficient of pure V02 samples at diffe- rent temperatures in the energy gap region.

T E M P E R A T U R E ( ! C I

FIG. 4.

-

Variation of the energy gap as a function of tempe- rature. The intercept of the tangent to the curve at 330 K is used

to obtain the conductivity activation energy.

the absolute magnitude being different because of a different definition of the gap (in their case, for lack of a better criterion, they arbitrarily took for E, the energy at which a = 4 000 cm-').

4. Optical absorption in V,-,Cr,02. -The remar- kable result here is that the different phase changes have little effect on the optical absorption. For example figure 5 shows a plot of optical transmission through a 4p thick V, -,Crx02 film as a function of temperature (X = 7.3 X 10-3). As one goes through the first order T-M2 transition, only a relatively small variation (20

%)

is observed at the phase transition. This is reminiscent of the small effect observed at this transition in the electrical resistivity p. A plot of p in the same range is shown for comparison. Figure 6 shows the absorption coefficient as a function of photon energy for three temperatures representative of the three different phases. One sees the small difference

10 20 30 40 50 5 5

--

=*

T E M P E R A T U R E *

FIG. 5.

-

Optical transmission and resistivity of an epitaxial V0.927e0.07302 layer as a function of temperature. The phase symbols T, M2 and R are noted on the corresponding parts

of the curves.

103L

3 4 6 6 7

P H O T O N ENERGY ( - V )

FIG. 6.

-

Absorption coefficient of an epitaxial V0.927Cr0.07302 layer versus photon energy in the three insulating phases Mz,

T and MI.

between the curves for the T phase and the M, phase. There appears to be a small energy shift (comparable to that observed in pure VO, between these two temperatures) plus an overall reduction of the absorption in the M2 phase. Also the energy shift between the M, and T curve is similar to what it is in pure VO, between the two considered temperatures ( W 0.1 eV). In general the only difference with pure V02 is that :

a) The absorption coefficients are a factor 4 higher. b) The temperature dependence of a is not as accu- rately described as a translation along the energy axis.

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04-62 P. MERENDA, D. KAPLAN AND C. SOMMERS perfect cubic symmetry ; the three levels of the triplet

lie within 0.06 eV of each other, while the doublet lies

1.8 eV above the triplet. Upon introducing the vanadium displacements of the insulating phases (M,, M,, T), this quasi-degeneracy is lifted, but the splitting of the triplet remains small (- 0.3 eV). We have also

made

a

calculation representative of a pair in the M, phase to estimate the degree of covalent bonding between the two atoms in the pair. For this we used a double octahedron cluster. The splitting between the bonding and antibonding orbitals is found to be 0.5 eV. It does not appear that intrasite transitions can explain the absorption edge for the following reasons :

a) The d-d transitions are almost forbidden (we find only weak hybridization of the d functions). b) The calculated splittings are smaller than the experimental gap.

c) If the gap was due entirely to a distortion induced level splitting, it should vary appreciably at the T-M2 transition where the distortion is modified. This is contrary to our observation.

An intrapair transition involving a transfer between bonding and antibonding orbitals is a priori possible, since we found appreciable covalent bonding. But this would have a strong polarization selection rule : the transition would be allowed for light polarized parallel to the pair direction (c, axis). As we said above, there is only a small polarization effect and it is in the oppo- site direction.

We thus think that intersite transitions, e. g. between chains, are involved in the absorption edge. The

[l] ADLER, D. and BROOKS, H., Phys. Rev. 155 (1967) 826. [2] Z~BERS~TEJN, A. and Morr, N. F., Phys. Rev. B 11 4383

(1975).

[3] POUGET, J. P., LAUNOIS, H., RICE, T. M., DERNIER, P., GOSSARD, A., V~LENEWE, G. and HAGENMULLER, P,,

Phys. Rev. B 10 (1974) 1801.

[h] DRILLON, M. and VILLENEWE, G., Mater. Res. Bull. 9 (1974) 1199.

[5] POUGET, J. P., LAUNOIS, H., D'HAENENS, J. P., MERENDA, P. and RICE, T. M., Phys. Rev. Lett. 35 (1975) 873.

[6] SLATER, J. B. and JOHNSON, K. H., Phys. Rev. B 5 (1972) 844. [7] SOMMERS, C., DE GROOT, R., KAPLAN, D. and ZYL-

BERSZTEJN, A., J. Physique Lett. 36 (1975) L-157. 181 GAVINI, A. and KWAN, C. C. Y., Phys. Rev. B 5 (1972) 3138. [9] VERLEUR, H. W., BARKER, A. S. Jr and BERGLUND, C. N.,

Phys. Rev. 172 (1968) 788.

energy E of such a transition contains a contribution U from the electron-electron correlation energy which should be little sensitive to distortion. In this case the optical band gap is closely related to the one pertaining to electrical conductivity. Assuming the two are identical, we can deduce from our data an activation energy for the conductivity. One has to take into account the temperature dependence of the band gap Eg. In a region where Eg(T) can be approximated in the form :

the conductivity activation energy for intrinsic conductivity is approximately Eg,/2. The data of figure 4 in the range 300 < T < 340 K yield as value Eg,/2 = 0.4 eV. In this temperature region the conductivity is intrinsic, as indicated by the behaviour of the thermoelectric power [2], and has an activation energy of 0.42 eV. The agreement is therefore quite good.

Finally one can discuss the nature of the wave- function in the final state of the optical transition. The lowest excited levels are n antiboncing triplet orbitals (the K * band in the terminology of Goodenough [13]).

However examination of the charge distribution in the cluster calculation shows only a small mixing of oxygen components, so that the corresponding matrix element should be quite small. The general shape of the absorption in the 0.6 to 1.7 eV range suggests transitions to a single relatively broad band centered at 1.45 eV. The D* band formed by the a antibonding

doublet orbitals appears to a likely candidate for this band, since the oxygen overlap is there appreciable.

1101 BORISOV, B. S., KORETSKAYA, S. T., MOKEROV, V. G.,

RAKOV, A. V. and SOLOV'EV, S. G., Sov. Phys. Solid

State 12 (1971) 1763.

[ l l ] LADD, L. A. et PAUL, W., Solidstate Commun. 7 (1969) 425. [l21 BENNETT, H. E. and KOEHLER, W. F., J. Opt. SOC. Am. 50

(1960) 1.

[l31 GOODENOUGH, J. B., J. Solid State Chem. 3 (1971) 490. [l41 RICE, T. M., BRINKMAN, W. F. and MCWHAN, D. B.,

Proceedings of the 10th Semiconductor Conference,

p. 293 (1970).

[l51 MAREZIO, M., MCWHAN, D. B., REMEIKA, J. P. and DER-

NIER, P., Phys. Rev. B 5 (1972) 254.

[l61 Since it is likely that stresses are present in the epitaxial layers, it is important to note that the (001) substrate orientation yields an isotropic stress in the ( 0 0 1 ) ~ plane of the layer, so that there should be no tendency to